Conventionally, seat frames for automobiles having three-dimensional structures have been produced by bending steel pipe members into desired three-dimensional shapes by benders and then welding the abutting ends of the steel pipe members.
However, according to such a conventional process, handling of the pipe members presents some difficulty if any attempt is made to automate the process, and ensuring a high precision level is also difficult particularly when the shape of the frame to be produced is complicated. Furthermore, since the pipe members tend to elastically restore their original shapes (or to spring back) after bent into desired three-dimensional shapes, some difficulties arise in welding two abutting parts of the pipe members together. Also, conveying the three-dimensional work to the succeeding stations involves some difficulty.
Japanese patent laid-open publication No. 60-54229 proposes an automated pipe working system, but this system requires readjusting of the jigs for supporting the pipe so many times that the system is not suitable for forming pipe frames having complicated three-dimensional structures.